J. Cent. South Univ. (2014) 21: 857-861

DOI: 10.1007/s11771-014-2010-8

Purification process of coal-based coke powder as anode for Li-ion batteries

YANG Juan(杨娟)^{1, 2}, MA Lu-lu(马路路)², ZHOU Xiang-yang(周向阳)²

1. Powder Metallurgy Research Institute, Central South University, Changsha 410083, China;

2. School of Metalllurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: A process of purification of coal-based coke powder as anode for Li-ion batteries was attempted. The process started with the treatment of coke powder with dilute hydrofluoric acid solution, followed by united-acid-leaching using sulfuric acid and hydrochloric acid. The effects of altering the hydrofluoric acid addition, hydrofluoric acid concentration, contact time, temperature and acid type were investigated. A minimum ash content of 0.35% was obtained when proper conditions were applied. The electrochemical performance of purified coke powder shows greatly improved electrochemical performance. The as-purified coke powder presented an initial reversible capacity of 257.4 mAh/g and a retention rate of 95% after 50 cycles. The proposed purification process paves a way to prepare a promising anode material with good performance with low cost of coke powder for Li-ion batteries.

Key words: purification process; coal-based coke powder; anode material; Li-ion batteries

1 Introduction

Coal-based coke powder is a by-product when coke is smashed for metallurgy and chemical industry. There are more than 40 million tons of coke powder every year in China, most of which is combusted as cheap fuel or abandoned directly because of its high mineral impurities and small grain diameter [1]. This causes both waste of resources and environmental pollution. Therefore, it is of great significance to develop new application of coal-based coke powder. In fact, coal-based coke powder appears to be a promising anode material for Li-ion batteries (LIB) from the viewpoints of low cost, available sources, environmentally friendly and has similar electrochemical performances with other carbonaceous anode materials such as graphite, carbon fiber and petroleum coke [2-6]. In spite of that purification is a key processing step to obtain coal-based coke powder for LIB. It is difficult to find many published papers related to the purification of coal-based coke powder, but several researches about the purification of natural graphite are already pending or published. Generally, it can be classified into two methods which are pyrometallurgical method [7-11] and hydrometallurgical method [12-18]. However, the purification process of coke powder is varied from those of graphite due to the different crystalline structures and micro-texture.

To develop improved and low-cost carbonaceous materials for LIB, we initially studied to purify coal-based coke powder by a hydrometallurgical process, named united-acid-leaching (UAL) method. The intensification of leaching with hydrofluoric acid was investigated along with the effect of acid type on removing impurities. An effort was also undertaken to correlate the electrochemical performances of cells with purified coke powder that initially contained different impurities.

2 Materials and methods

2.1 Materials

The coal-based coke powder used in this work was obtained from Shanxi Province, China. Proximate analysis of coke powder is shown in Table 1. The as-received sample has an average particle size of 500 μm. All the reagents including hydrofluoric acid, sulfuric acid and hydrochloric acid used for leaching were of analytical grade.

Table 1 Proximate analysis of coke powder

2.2 Procedures

The dried powder was ground and sieved, and particles below 44 μm were used in this work. A portion of the ground coke powder (10 g) and a predetermined volume of dilute hydrofluoric acid solution or mixed aqueous solution containing different portions of hydrofluoric acid, sulfuric acid and hydrochloric acid were charged in a dissolution container equipped with a rotary stirrer. During the leaching process, the entire suspension was maintained at a certain temperature and stirred at a rate of 200 r/min. After the required time, the solid was filtered, washed with copious amounts of water, and dried at 120 °C for 24 h.

The optimal purification parameters were determined by evaluating the effects of the following factors: hydrofluoric acid addition, hydrofluoric acid concentration, contact time, temperature and acid type.

2.3 Analysis

Determination of proximate analysis was according to China National Standard GB 2001—91 documents [16]. The ash content of the coke powder was calculated using the following equation:

w_ash=m₁/m×100%                           (1)

where w_ash is the ash content m is the initial mass of coke powder and m₁ is the remaining mass of the burned residue.

IRIS Advantage 1000 ICP-AES (Thermo Fisher Scientific Inc.) was used to analyze the elements remained in the coke powder, which were not removed by purification. Electrochemical measurements were performed in a three-electrode system in which lithium metal was used as both the counter and the reference electrodes. The electrolyte used was 1 mol/L LiPF₆ dissolved in a 50:50 (V/V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were assembled in an Ar filled glove box in which H₂O concentration was kept below 1×10^-6. Electrochemical measurements of the cells were carried out on LAND CT-2001A (Wuhan Jinnuo Electron Co. Ltd.)

3 Results and discussion

3.1 Effect of hydrofluoric acid leaching process on purification

The method outlined in this work can be used to remove impurities from coal-based coke powder so that the purified coke powder can be employed as anode material for LIB. In the following section, various experimental parameters are examined.

Figure 1 shows the variation of ash content with changing hydrofluoric acid addition. As shown here, the ash content decreased slightly with increasing hydrofluoric acid addition when the hydrofluoric acid concentration was kept as low as 4%. This may be attributed to that it is hard for hydrofluoric acid to diffuse into the coke particles under low acid concentration, indicating that hydrofluoric acid concentration may have great effect on the purification process.

Fig. 1 Variation of ash content with changing HF acid addition (HF concentration: 4%, contact time: 60 min, room temperature)

Figure 2 shows the effect of hydrofluoric acid content on the ash content of purified coke powder. The result showed the ash content decreased rapidly with increasing hydrofluoric acid content in the solution. Therefore, it is important to increase hydrofluoric acid content if a higher efficiency of removing impurities is desired. When the hydrofluoric acid content was below 19%, ash content decreased from 2.78% to 0.91% with increasing hydrofluoric acid content. Further increase of hydrofluoric acid content had only a slight effect on ash content.

Fig. 2 Variation of ash content with changing HF content (HF acid volume: 30 mL, contact time: 60 min, room temperature)

Figure 3 shows the variation of ash content with changing contact time. It is apparent from the figure that ash content decreased with prolonging time and then almost remained constant after 50 min, revealing a fast reaction speed. It was concluded that a contact time of 60 min was sufficient.

Figure 4 shows the effect on ash content of varying temperature. As can be seen in the figure, the ash content increased monotonously with temperature. Clearly, temperature has a negative effect on the efficiency of purification. This may be ascribed to a decrease in the hydrofluoric acid content in solution when the volatilization of hydrofluoric acid is accelerated under high temperature. As previously explained, low hydrofluoric acid content could reduce the efficiency of purification. Hence, the temperature of leaching with hydrofluoric acid content should be kept at room temperature.

Fig. 3 Variation of ash content with changing contact time (HF content: 24%, HF acid volume: 30 mL, room temperature)

Fig. 4 Variation of ash content with changing temperature (HF content: 24%, HF acid volume: 30 mL, contact time: 60 min)

An ash content as low as 0.89% was obtained with a hydrofluoric acid content of 19%, in 30 mL acid solution after 60 min at room temperature. However, this purity of coke powder still can’t fulfill the requirement of anode material for LIB, the ash content of which is often less than 0.5%.

3.2 Effect of other acid on removing impurities

To further purify the coke powder and investigate the effect of acid type on the purification process, sulfuric acid and hydrochloric acid were employed.Table 3 gives the mass fraction of impurities in coke powder obtained after various chemical processing. As listed in Table 2, most of the mineral impurities could be dissolved in dilute hydrofluoric acid and the mass fraction of most impurities in coke powder declined substantially after HF-leaching. However, there are still quantities of some elements remained including Si and Al which may be concealed in the carbon lattice, and Ca and Mg which form precipitations when react with hydrofluoric acid. These remaining elements contribute to the substandard ash content. Thus, it is essential to carry out UAL process for further purification.

It was observed that addition of sulfuric acid and hydrochloric acid both had significant positive effects on the purification, although the solubility and selectivity of each acid are different. The sample obtained by chemical process in HF+H₂SO₄+HCl produced the lowest ash content of 0.35%. This sample showed similar chemical compositions for ash as the pristine coke powder, indicating that small amount of the mineral impurities in the coke particles were not removed by purification. From these observations, we concluded that this applied UAL process is applicable to purify the coal-based coke powder and ensures a thorough purification of the coal-based coke powder for LIB use.

Table 2 Mass fraction of impurities of coke powder after different chemical processes (HF content: 24%, HCl content: 24%, H₂SO₄ content: 24%)

3.3 Characterization of obtained powder

Figure 5 shows the SEM images of coke powder derived from as-received and after milling samples. The image of Fig. 5(b) clearly shows only a small variation in the shape, which is evident in the different particles shown in Fig. 5(a).

Fig. 5 SEM micrographs of coke powder derived from as-received (a) and after milling (b) samples

Figure 6 shows the X-ray diffraction patterns of the coke powders before and after purification, indicating that both the patterns show almost the same broadening diffraction peaks of soft carbon crystals. This exhibits that the modifying process of purification on coke does not damage its essential structure, which is favorite in the anode for lithium ion batteries. Figure 7 shows the Galvanostatic lithium intercalation and de-intercalation curves of coke powder before and after purification. From the figure, it is clear that the two charge-discharge curves are quite similar in shape. This once again reflects that the purification does not change general structure of coke powder. Both carbons exhibit a typical soft carbon voltage profile in which the potential slowly decreases. However, the purified coke powder shows a comparatively longer slope as compared with that of the pristine coke sample. It is evident that the capacity of the purified coke powder has increased by approximately 10%, indicating that the initial charge–discharge performance of treated coke powder has been improved in comparison with the untreated one.

Figure 8 shows the plots of the capacity as a function of cycle number for the coke powder before and after purification, indicating that the reversible capacities of purified coke electrode are higher than that of pristine coke electrode for all cycles. The cycling performance curve of purified coke electrode presents smooth and stable state after the sixth cycle, while capacities of unpurified coke show continuous drop with the increase of cycle number. The reversible capacities of 244.5 and 209.7 mAh·g^-1 after 50 cycles can be retained respectively with the purified and unpurified electrode, revealing great improvement of electrochemical performance after the united-acid-leaching process for coal-based coke powder.

Fig. 6 XRD patterns of coke powder before and after purification

Fig. 7 Galvanostatic lithium intercalation and de-intercalation curves of coke powder before and after purification

Fig. 8 Cycle number vs discharge capacity plots of coke powder before and after purification

4 Conclusions

In the present work, an effective purification method is developed to purified coal-based coke powder for LIB use. The purified coke powder is gained by treating pristine coke powder in room temperature container using a united-acid-leaching method. A minimum ash content of 0.35% is obtained when proper conditions are applied. The electrochemical performance of purified coke powder shows greatly improved electrochemical performance. Therefore, this approach shows that the coke powder purified by the proposed process can become a promising anode material with excellent performance price ratio for rechargeable Li-ion batteries.

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(Edited by HE Yun-bin)

Foundation item: Projects(51274240, 51204209) supported by the National Natural Science Foundation of China; Project(2012M521545) supported by the National Postdoctoral Science Foundation of China

Received date: 2012-10-10; Accepted date: 2013-01-25

Corresponding author: ZHOU Xiang-yang, Professor, PhD; Tel/Fax: +86-731-88836329; E-mail address: yjjxgz@126.com